1. Field of the Invention
The invention relates to an evaluation method and an evaluation device for output signals of an integrated circuit, especially of a digital data signal of a semiconductor storage circuit.
2. Description of the Related Art
In the past, advances in manufacturing integrated circuits have resulted in exponential increase of the clock frequencies. In addition, the data transfer speed of these circuits increased exponentially. Thus, particularly in state-of-the-art semiconductor memories, data transfer rates in the range of 1 Gigabit per second already occur on a data transfer channel.
However, in semiconductor storage circuits operated with high frequencies interfering factors increasingly occur, such as signal cross-talk or electromagnetic pulses rendering detection of a signal difficult.
By internal signal processing of an integrated circuit, signals are processed in a preset manner and output as defined output signals. The amplitude and the behaviour of the output signal of the integrated circuit are defined by corresponding reference voltage levels in the associated component specification. Upon reading the output signal, by way of comparison of the measured voltage level of the output signal to the reference voltage level, it can be verified if the output signal is above or below the reference voltage preset by the specification, respectively. As a result of this comparison, it is decided if a digital output signal represents a logical “1” or a logical “0”. Due to various influences, which may occur in a real system, short-term signal variations can occur. This in turn can result in errors in interpretation of the corresponding signals. Therefore, it is necessary to identify such erroneous signals and the optionally also erroneous circuit portions of the integrated circuit. Usually, this is effected with the aid of a compliance test.
In a conventional compliance test an attempt is made to sample the signal with high accuracy in order to obtain an idea of the signal behaviour as accurately as possible. The individual measurement points are stored. Subsequently, the region in which the data eye is opened is determined by way of the measurement data. For testing the signal, it is verified if the mask defined by the component specification fits in the data eye reconstructed from the measurement data. Since the accuracy of such a test method is primarily determined by the amount of acquired measurement data, inevitably, it is necessary to acquire the signal with a sampling frequency and resolution as high as possible. Due to the technical and temporal effort required for such a measurement, the conventional test method is not suitable for the production in high volume. Rather, up to now, the compliance test is performed in a separate procedure succeeding the production. This requires additional test time.
However, detailed acquisition of the time/voltage values is only reasonable in case of characterization of a component, especially, if certain characteristics of the component are to be analyzed by the exact analysis of the data eye. However, for production in high volume, this type of analysis is not required. Here, a simple “pass/fail” result is sufficient, with which a statement is possible of whether the output signal of the concerned component corresponds or not to the specification.